The production of pharmaceutically applicable or enzymatically active proteins is a key area in the rapidly developing biotechnology industry. Since the beginning of the era of recombinant DNA technology a great number of valuable heterologous proteins have been produced in and secreted from eukaryotic host cells which had been transformed with suitable expression vectors containing DNA sequences coding for said proteins. One of the major problems with the production of secreted proteins in eukaryotic expression systems is to avoid malfolded biologically inactive product.
It is now generally accepted that proteins destined for secretion from eukaryotic cells are translocated to the endoplasmic reticulum (ER) due to the presence of a signal sequence which is cleaved off by the enzyme signal peptidase located in the rough ER membrane. The protein is then transported from the ER to the Golgi and via Golgi derived secretory vesicles to the cell surface (S. Pfeffer and J. Rothman, Ann. Rev. Biochem. 56:829-52, 1987). Another major step in the production of correctly processed and correctly folded proteins is the conversion of proproteins to the mature forms in the Golgi apparatus and secretory vesicles. The cleavage of the proprotein occurs at a so-called dibasic site, i.e. a motif consisting of at least two basic amino acids. The processing is catalysed by enzymes located in the Golgi-apparatus, the so-called "dibasic processing endoproteases".
There are different "dibasic processing endoproteases" known which are involved in the processing of protein precursors, for example the mammalian proteases furin, PC2, PC1 and PC3, and the product of the yeast YAP3 gene and yeast yscF (also named KEX2 gene product; herein referred to as KEX2p).
KEX2p is involved in the maturation of the yeast mating pheromone .alpha.-factor (J. Kurjan and I. Hershkowitz, Cell 30:933-943, 1982). The .alpha.-factor is produced as a 165 amino acid precursor which is processed during the transport to the cell surface. In the first step, the 19-amino acid signal sequence (pre-sequence) is cleaved off by the signal peptidase. Then the precursor is glycosylated and moves to the Golgi where a 66-amino acid pro-sequence is cut off by KEX2p. The .alpha.-factor pre-pro-sequence is also known as .alpha.-factor "leader" sequence. A second protease in the Golgi apparatus, i.e. the KEX1 gene product, is responsible for the final maturation of the protein.
KEX2p is encoded by the KEX2 gene and consists of a N-terminal catalytic domain, a Ser/Thr rich domain, a membrane-spanning domain and a C-terminal tail responsible for Golgi localization. Mutant KEX2p enzyme lacking 200 C-terminal amino acids, including the Ser/Thr rich domain, the membrane spanning domain and the C-terminal tail, still retains KEX2p protease function, viz. cleavage at the C-terminal side of a pair of basic amino adds, such as Lys-Arg or Arg-Arg [Fuller et al., 1989, Proc. Natl. Acad. Sci. 86, 1434-1438; Fuller et al., 1989, Science 246, 482-485].
Leader sequences such as the yeast a-factor leader sequence are widely used for the production of secreted heterologous proteins in eukaryotic cells. In many cases, however, great difficulties are encountered because considerable amounts of biologically inactive proteins are produced due to malfolding and aggregation of the proteins, especially in the case of low molecular weight proteins.